Tuesday, August 16, 2011

CT Scans of Bennettazhia

Posted by
Michael Habib

Back in 2008, I had CT scans done of the humerus of Bennettazhia oregonensis. I have one of the slice videos here, which I have previously only shown in talks (though they did make an appearance in two television programs). The outer, bright layer is the cortical bone (very thin). The grey areas are matrix. You can see some of the "trabecular" struts running through the matrix.

Originally described as “Pteranodon” oregonensis by C. W. Gilmore in 1928, Bennettazhia oregonensis consists of a single well-preserved and uncrushed humerus, and two associated dorsal vertebrae. Nessov (1991) erected a new genus name for the specimen, and reclassified the specimen as an azhdarchid in agreement with Bennett (1989). The humerus retains a deltopectoral crest with a primitive, unwarped orientation. The crest is highly elongated, and curves anteroventrally, which is consistent with the humeral morphology of azharchoids. The specimen is currently housed at the Smithsonian's National Museum of Natural History in Washington, DC. (USNM 11925)

The affinities of Bennettazhia have been somewhat uncertain, and Bennett (1994) suggested that it may be a dsungaripterid (though the definition of the Dsungaripteridae differed at the time from the current usage). While dsungaripterid humeri show some similarities to those of azhdarchoids (such as retention of an unwarped deltopectoral crest), dsungaripterids differ markedly from azhdarchoids in having thickened cortical bone and apneumatic appendicular elements (Fastnacht, 2005). These differences are easily visualized using computed tomography (CT) imaging. By utilizing high resolution cross sectional images derived from CT scans in 2008, I confirmed that Bennettazhia has extremely thin-walled bones with significant trabecular cross-bracing, which is consistent with azhdarchoid morphology and inconsistent with the structural characteristics of dsungaripterids. This still represents (so far as I am aware) the only case of CT imaging being used to assess the phylogenetic position of a pterosaur.

The use of CT imaging also allowed for detailed analyses, both quantitative and qualitative, of the structure and biomechanical properties of the proximal forelimb of Bennettazhia. Because the humerus of Bennettazhia is very well preserved and uncrushed, a significant amount of information can be gleaned regarding its flight dynamics, despite the fact that only a single appendicular element is available for analysis. The shape and direction of internal trabecular bracing within the deltopectoral crest is of particular interest. The Bennettazhia study represents an example of how CT imaging and structural analysis can be used to extract the maximum amount of information from sparse remains. The ability to maximize information potential from limited fossil material is of special importance to pterosaurs, many of which are known from a limited number of specimens.

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This blog is dedicated to discussions and news related to vertebrate paleontology, particularly animals from the Mesozoic (dinosaurs, pterosaurs, and others). This blog is the work of two paleontologists from U.S. West Coast: it is the work of Justin Hall and Michael Habib, both at the University of Southern California in Los Angeles.

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